Neuroscience LC1 Introduction to Neuroanatomy PDF

mathematical modeling, and psychiatry to COURSE OUTLINE understand the fundamental and emergent properties of neurons and neural cir...

mathematical modeling, and psychiatry to COURSE OUTLINE understand the fundamental and emergent properties of neurons and neural circuits I. Introduction to Neuroscience II. Nervous System Main Divisions A.Central Nervous System B.Peripheral Nervous System III. Processes of the Nervous System A.Sensory Input B.Integration C.Motor Output 1. Somatic NS 2. Autonomic NS a) Sympathetic Division b) Parasympathetic Division IV. Order of Neuron (Motor)/2 Neuron System A.Upper Motor Neuron B.Lower Motor Neuron V. Neurobiology of Neurons and Neuroglial Cells A.Cerebral Hemodynamics B.PNS C.Nervous System D.2 Main Cells of the Nervous System 1. Nerve Cells Figure 1. The Nervous System 2. Neuroglial Cells VI. Structure of Neuron A.Parts of Neuron II. NERVOUS SYSTEM MAIN DIVISIONS 1. Cell Body 2. Dendrites A. CENTRAL NERVOUS SYSTEM 3. Axons B.Ultrastructure of Neuron Brain (protected by the skull) 1. Nucleus Spinal cord (protected by the spine/) 2. Cytoplasm Contains Relay Neurons (Interneurons, C.Neuronal Bodies association neurons) 1. Cortex Brain and spinal cord with their associated 2. Inner Grey Matter of Spinal Cord tracts and nuclei 3. Nuclei in CNS 4. Ganglia in PNS VII. Conduction Velocity A.Unmyelinated Fibers B.Myelinated Fibers C.Nodes of Ranvier VIII. Neuronal Synapse A.Types of Neuronal Synapse IX. Neuronal Transmission A.Electrical Transmission B.Synaptic Transmission 1. Types of Synaptic Transmission X. Basic Tenets of Synapses XI. Six Tenets of the Neuron Doctrine XII. Types of Neurons XIII. Functional Classification of Neurons XIV. Neuroanatomy Basics A.Anatomical Classification of Neurons B.Size/Length Classification of Axon 1. Golgi Type I 2. Golgi Type II XV. Projection Fibers I. NEUROSCIENCE Figure 2. Divisions of the Nervous System Also known as “Neurobiology” and is the scientific study of the nervous system. It is a multidisciplinary branch of biology that combines physiology, anatomy, molecular biology, developmental biology, cytology, BATCH 2028 1F 1 NEUROSCIENCE LC 1 Afferent- sensory nerve fibers (“arrives, ascends”) Somatic (somatosensory; e.g., skin) sensory fibers Visceral (viscerosensory; e.g., stomach/internal organs) sensory fibers Impulse is conducted from receptors towards the CNS B. INTEGRATION Processing of information C. MOTOR OUTPUT Figure 3. Main Parts of the Nervous System Effector organ Efferent - motor nerve fibers (“exits”) B. PERIPHERAL NERVOUS SYSTEM Conducts impulses away from the CNS towards (Made up of Nerves and ganglia: outside of the the effector muscles (skeletal/smooth/cardiac) brain and spinal cord (CNS)) and glands Composed of Cranial Nerves, Spinal Nerves, Voluntary (conscious) somatomotor and Peripheral Nerves. o Skeletal muscles All cranial and spinal nerves and their Involuntary/autonomic associated roots/ nerves and ganglia o Cardiac, smooth muscles, glands (e.g., 12 pairs of cranial nerves (innervation of heart rate, salivation, digestion, breathing, head/face) sexual arousal, tears) 31 pairs of spinal nerves (innervation of the TWO PARTS OF THE PNS: rest of the body) ○ including the peripheral nerves (e.g., ulnar 1. Somatic NS – somatomotor, voluntary, conduct nerve, peroneal nerve, sciatic nerve, etc. impulses from CNS-> skeletal muscles ○ innervation of individual muscle, muscle 2. Autonomic NS – visceromotor, involuntary, group, or glands) conducts impulses from CNS -> cardiac and No bony protection smooth muscles, glands Contains Sensory and Motor Neurons a) Sympathetic division – mobilize body systems during activity, “fight/flight” b) Parasympathetic – conserve energy, promote house-keeping functions during rest, “rest and digest” IV. ORDER OF NEURON (MOTOR)/ 2-NEURON SYSTEM Figure 4. Impulse flow - “2 neurons talking to each other” The relationship of afferent sensory A.UPPER MOTOR NEURON stimuli to the memory bank, correlation and Brain cortex towards the spinal cord coordinating centers, and common efferent o Corticospinal tract - control movement pathway. of extremities (cortex-spine) o Corticobulbar tract - control movement of muscles in the face (cortex-brain stem) B.LOWER MOTOR NEURON Starts at the brainstem and spinal cord nucleis o Cranial nerves (12 pairs) - starts at brainstem nuclei o Spinal nerves (31 pairs) – from ventral spinal cord nuclei Figure 5. Functions of the 12 Cranial Nerves III. PROCESSES OF THE NERVOUS SYSTEM A. SENSORY INPUT Monitoring stimuli occurring outside the body (receptors) Batch 2028 1F 2 NEUROSCIENCE LC 1 Broca’s area which are centers for language and speech on the left side will have greater neurons as compared to the right side) o Each region (100B) has thousands of interconnections= 1—trillion synaptic connections (varies with each individual) o Very complex form of communication Note: o neurons can be found even in the viscera Figures 6 and 7. Order of Neurons (intramural, postganglionic neurons of the ANS) o we may have roughly the same no. of V. NEUROBIOLOGY OF NEURONS neurons but we differ in the no. and AND NEUROGLIAL CELLS complexity of synaptic connections A. CEREBRAL HEMODYNAMICS D. TWO MAIN CELLS OF THE NERVOUS Brain: SYSTEM 2-3% of total body weight 1. NERVE CELLS: neuron 2. NEUROGLIA CELLS: glial/supporting cells 15% of cardiac Cerebral blood flow: ○ 10x more abundant than neurons output 50ml/100gm/min 20% of O2 250-500ml of O2/min consumption 25% of glucose 75-100 ml of glucose/min use The brain is metabolically active, these data increase with activity. B. PNS Table 1. PNS (Cranial/Spinal) Sensory (afferent Motor (efferent division) division) Somatic Visceral Somatic Visceral Gen: Gen.: Gen.: Gen.: Motor Touch, Stretch, Motor innervation pain, pain, innervati of smooth pressure, temp., on of all muscle, vibration, chemical skeletal cardiac & changes, muscles muscle, and proprioce & glands; ption in irritation equiv. to skin, in ANS Figures 8 and 9. Cell Types in the Nervous System body viscera; wall, and nausea, 1. NERVE CELLS: NEURON limbs & hunger Structural and functional unit of the nervous Sp.: Sp.: -Parasymp system Hearing, Taste athetic Can survive for a lifetime (limited by equilibriu division diseases. Trauma, stroke, etc) m, vision, -Sympathet NO [power of regeneration one cell body is smell ic division dead C. NERVOUS SYSTEM Specialized to conduct electrical signals/impulses thru its membrane The most complex system in the human Communicate with other neurons via a body chemical synapse A network of approximately 100 billion ❖FUNCTIONS: neurons at birth - most basic function: COMMUNICATION o Neuron density differs in different regions - Neuron-neuron: via a chemical of the brain (depends on synapse function/eloquence) - When stimulated, neurons produce electrical (e.g. 1) The precentral gyrus has a nerve impulses (can be electronically greater number of neurons than the recorded and measured as an action prefrontal cortex and 2) Wernicke and Batch 2028 1F 3 NEUROSCIENCE LC 1 potential) that travel through its membrane then… A. PARTS OF A NEURON - To activate or inhibit them or conduct chemical synapse to an effector organ (neuromuscular junction with muscle cells or neuroglandular junction with glands, to inhibit of activate them Figure 12. Neuron in the PNS, observe the presence of Schwann cell (Neuroglia cell) 1. CELL BODY The expanded portion of the neuron that contains the nucleus No centrioles, incapable of cell division (amitotic) Basophilic, due to the abundance of rough endoplasmic reticulum (RER) and polyribosomes ○ Clumps or RER & polyribosomes are Figure 10. Types of Synapses referred to Nissl bodies (site of protein synthesis) Function of a neuron: Receptive : receive stimuli (dendrites) Integrative : processing impulse on the higher center (body) Conductive : initiating action potential and transduce the impulse to an effector via a synapse (axon) Cell to cell signaling or synaptic communications: Progress during development and growth How do you develop language, skills, cognition, talent, memories, emotions, etc., as you age. Figure 13. Motor neuron with Nissl bodies VI. STRUCTURE OF A NEURON Three main parts: 2. DENDRITES ○ Cell body (perikaryon/soma) Short, tapering, diffusely branched with a nucleus (karyon) processes ○ A single axon (efferent) Receptive input region ○ Multiple dendrites (afferent) Electrical signals are conveyed as graded ○ numerous dendritic spines potential towards the body 5-150 um Always unmyelinated Long-lived With cytoplasm devoid of golgi complex o could last >100 years depending on Contain nissl bodies in their proximal parts individual age and thus the initial portions of dendrites Amitotic- not capable of cell division stain basophilic High metabolic rate Often have small protrusions, called Neuronal cell membrane dendritic spines, that expand the dendritic o Electrical signaling surface area and serve as sites of synaptic contact with the axon of another neuron Dendritic spine ○ Points of synapse with other neurons Figure 11. Neuron in the CNS, observe the presence of Figure 14. Dendritic Spines oligodendrocyte (Neuroglia cell Batch 2028 1F 4  NEUROSCIENCE LC 1 3. AXONS Autonomic ganglias (can be intramural, Typically, one per neuron within the viscera) Generate nerve impulses (action potential) Cranial nerve ganglia (trigeminal ganglia) away from soma Originate from axon hillock to distal portion-axon terminal-end bulbs-synapse with one neuron Devoid of ribosome Single process may be up to >100 cm in length When bundled together ○ CNS: tracts Axon= fiber tracts or simply fivers E.g., corticospinal tract, spinothalamic tract, DCML, etc. Figure 16. Cerebral Cortex ○ PNS: nerves Axon = nerve fiber or simply fibers E.g., facial nerve, median nerve, femoral nerve, s2 spinal nerve, etc B. ULTRASTRUCTURE OF A NEURON 1. NUCLEUS Large, spherical to ovoid, centrally located Single prominent nucleolus Finely dispersed chromatin Transcriptionally active (protein synthesis) Figure 17. White Matter in Brain Figure 15. Ultrastructure of a Neuron 2. CYTOPLASM Polyribosomes with abundant RER (Nissl Figure 18. CNS: Cerebral Cortex; Nuclei: Thalamus, Basal bodies) Ganglia o Protein synthesis Golgi apparatus o Protein secretion Lots of mitochondria, most abundant in axon hillock Extensive cytoskeleton for protein transport o Microfilament o Neurofilaments No centriole o Neurons do not undergo cell division C. NEURONAL BODIES OR SOME GROUPINGS/CLUSTERINGS 1. CORTEX (OUTER GREY MATTER) 6 layers in the cerebral cortex Figure 19. CNS: Inner Grey of the Spinal Cord 3 layers in the cerebellar cortex 2. INNER GREY MATTER OF SPINAL CORD (DORSAL AND VENTRAL HORNS) 3. NUCLEI IN CNS Subcortical region ○ Thalamus ○ Basal ganglia ○ Midbrain ○ Brainstem (pons and medulla) Cell bodies form irregular clusters 4. GANGLIA/GANGLION IN PNS Figure 20. PNS: Dorsal Root Ganglia Found in dorsal root ganglia Batch 2028 1F 5  NEUROSCIENCE LC 1 Found in white matter ○ E.g., Corpus callosum, corticospinal tract, spinothalamic tract, DCML pathway) Connective tissue is absent 2. NERVES Bundles of axons that extend out from the brainstem as cranial nerves and Figure 21. PNS: Trigeminal Ganglia from the spinal cord as spinal nerves (PNS) Surrounded by connective tissue sheaths VII. CONDUCTION VELOCITY Depend on the extent of myelination: A. UNMYELINATED FIBERS o No nodes of Ranvier o Continuous conduction o Slower conduction B. MYELINATED FIBERS o Gap of myelin sheath (NODES OF RANVIER) o Saltatory conduction o Faster conduction C. NODES OF RANVIER (NEUROFIBRIL NODES) ○ Myelin sheath gaps between adjacent Schwann cells, occur at regular intervals ○ Sites where axon collaterals can emerge Figure 22. Ganglia: Autonomic Ganglia ○ Unmyelinated axons- Schwann cells enclose axons but no myelin present Figure 23. Ganglia: Ganglion Cell DEFINITIONS Figure 25. Nodes of Ranvier Nodes of Ranvier are covered by myelin sheath, one segment is an axonal segment. The impulse will be jumping from node to node thereby causing a faster conduction. Figure 24. Corticospinal tract, pyramidal tract, upper motor neuron, lower motor neuron 1. TRACTS A bundle of axons (nerve fibers) within the CNS Batch 2028 1F 6 NEUROSCIENCE LC 1 TABLE 2. MYELINATED VS UNMYELINATED AXONS/FIBERS CLASSIFICATION TYPE OF NERVE EXAMPLE RELATIVE RELATIVE MYELINATION FIBER DIAMETER CONDUCTION VELOCITY A alpha (Aα) Α Motoneurons Largest Fastest Yes Sensory and A beta (Aβ) Touch, pressure Medium Medium Yes motor neuron A gamma (Aγ) Γ motor neurons to muscle Medium Medium Yes spindles (intrafusal fibers) A delta (Aδ) Touch, pressure, Small Medium Yes temperature, fast pain B Preganglionic autonomic Small Medium Yes nerves C Slow pain; postganglionic Smallest Slowest No autonomic nerves; olfaction Ia Muscle spindle afferents Largest Fastest Yes Sensory Only Ib Golgi tendon organ afferents Largest Fastest Yes II Secondary afferents of Medium Medium Yes muscle spindles; touch, pressure III Touch, pressure, fast pain, Small Medium Yes temp. IV Pain, temp., olfaction Smallest Slowest No Summary: For sensory, the muscle spindle afferents are the fastest because they are all myelinated and also the largest while the a-motoneurons are the motor neurons that conduct muscle contraction and are also very fast. The dendrites are highly branched with several dendritic spines that means it receives a lot/thousands of information it may be inhibitory or excitatory but it can process only a single information where it can be conveyed towards the axon. Figure 26. Multipolar Neuron The conduction process happens at the plasma-membrane not inside the cell. The propagation of the impulse of action-potential may only be generated because of the protein synthesis that happens only on the axonal body of the karyon. TABLE 3. GRADED POTENTIALS VS ACTION POTENTIALS GRADED POTENTIALS ACTION POTENTIALS Depending on stimulus, graded potential can be Action potentials always lead to depolarization and depolarizing or hyperpolarizing reversal of the membrane potential Amplitude is proportional to the strength of the stimulus Amplitude is all-or-none; strength of the stimulus is coded in the frequency of all-or-none action potentials generated Amplitude is generally small (a few m/V to tens of m/V) Large amplitude of ~100 mV Duration of graded potentials may be a few milliseconds Action potential duration is relatively short; 3-5 ms to seconds. Ion channels responsible for graded potentials may be Voltage-gated Na+ and voltage-gated K + channels ligand-gated (extracellular ligands such as are responsible for the neuronal action potential neurotransmitters), mechanosensitive, or temperature Batch 2028 1F 7 NEUROSCIENCE LC 1 sensitive channels, or may be channels that are gated by cytoplasmic signaling molecules The ions involved are usually Na+ , K + , or Cl The ions involved are Na+and K + (for neuronal action potentials) No refractory period is associated with graded potentials Absolute and relative refractory periods are important aspects of action potentials Graded potentials can be summed over time (temporal Summation is not possible with action potentials (due summation) and across space (spatial summation). to the all-or-none nature, and the presence of refractory periods) Graded potentials travel by passive spread (electrotonic Action potential propagation to neighboring membrane spread) to neighboring membrane regions regions is characterized by regeneration of a new action potential at every point along the way Amplitude diminishes as graded potentials travel away Amplitude does not diminish as action potentials from the initial site (decremental). propagate along neuronal projections (non-decremental). Graded potentials are brought about by external stimuli Action potentials are triggered by membrane (in sensory neurons) or by neurotransmitters released in depolarization to threshold. Graded potentials are synapses, where they cause graded potentials in the responsible for the initial membrane depolarization to postsynaptic cell. threshold. In principle, graded potentials can occur in any region of Occur in plasma membrane regions where voltage the cell plasma membrane, however, in neurons, graded gated Na+ and K + channels are highly concentrated. potentials occur in specialized regions of synaptic contact with other cells (post-synaptic plasma membrane in dendrites or some), or membrane regions involved in receiving sensory stimuli D. Dendrodentritic - rare (dendrite of one VIII. NEURONAL SYNAPSE neuron and dendrite of another neuron) DEFINITIONS Dendrites - Receptive region (graded potential towards cell body) Axon - Conducting region (action potential away from cell body) Synapses - Sites of impulse transmission; Convert electrical signal into chemical signal and permit neurons to communicate (Chemical Transmission) Figure 28: Different Types of Synapses A Figure 29: Different Types of Synapses B IX. NEURONAL TRANSMISSION/ Figure 27. Synapse A SIGNALING A. ELECTRICAL TRANSMISSION (IONIC) A. TYPES OF NEURONAL SYNAPSE One way transmission along the cell membrane A. Axondentritic - most common (dendrite-body-axon) B. Axosomatic - between the axon and the Flow of ions along the length of a membrane body of another neuron Graded potential (along dendrites towards the C. Axoaxonic - between the axon of one soma) neuron and axon of another neuron Action potential (along axons) Batch 2028 1F 8 NEUROSCIENCE LC 1 o Occurs when the excitable cell is stimulated, is a reversal of the resting membrane potential such that the inside of the cell membrane becomes positively charged compared with the outside. o The diffusion of ions through these channels changes the charge across the cell membrane and produces an action potential. An action potential lasts from approximately 1 millisecond to a few milliseconds, and it has two phases: (1) depolarization and (2) repolarization. o Before a neuron or a muscle fiber is stimulated, the gated Na and K Ion channels are closed. When the cell is stimulated, voltage-gated Na channels open and Na diffuses into the cell. The positively charged Na makes the inside of the cell membrane depolarized (more positive). If the depolarization causes the membrane potential to reach a threshold, an action potential is triggered. Threshold is the membrane potential at which gated Na channels open. o The depolarization phase of the action potential is a brief period during which further depolarization occurs and the inside of the cell becomes even more positively charged. As the Inside of the cell becomes positive, this voltage change causes additional permeability changes Figure 31: Synaptic Transmission in the cell membrane, which stop depolarization and start repolarization. o The repolarization phase is the return of the membrane potential to its resting value. It occurs when ligand-gated Na channels close and gated K channels open. When K moves out of the cell, the inside of the cell membrane becomes more negative and the outside becomes more positive. The action potential ends, and the resting membrane potential is re-established by the sodium-potassium pump. B. SYNAPTIC TRANSMISSION (NEUROTRANSMITTER) More diffuse/diffuse across space Note: Within the membrane of the cell it is an ionic or Chemical transmission (NT: E, NE, ACH) electrical transmission but it will be converted to a Cell to cell/ signal between cells chemical transmission once they reach the synapse. Axon terminals (e.g., axodendritic/ neuromuscular 2. Neuromuscular Junction junction/ neuroglandular) Peripheral nerve axon terminal ↓ Muscle cell Figure 30: General Neuron Communication TYPES OF SYNAPTIC TRANSMISSION: 1. Axodendritic Figure 32: Neuromuscular Junction Axon terminal of one neuron (Presynaptic Neuron) X. BASIC TENETS OF A SYNAPSE ↓ 1. The nerve impulse (in the membrane) can be electronically recorded and measured as an Dendrite (Particularly the dendritic action potential. The action potential, a wave of spines) of another neuron electrical depolarization, propagates along the (Postsynaptic Neuron) surface of the neuronal membrane to end feet Batch 2028 1F 9 NEUROSCIENCE LC 1 (axon terminal) at the axonal tips. The end feet 5. A synapse acts as a one-way valve. At a synapse, form synapses on a dendrite, perikaryon, or an impulses flow from the incoming axon to the next axon of one or more neurons, or on an effector neuron (not vice versa). cell. A synapse consists of: A.A presynaptic membrane provided by an axonal end foot B.A synaptic cleft C.A postsynaptic membrane of a neuron or effector cell 2. The branching of dendrites increases the surface area of any given neuron to receive a larger Figure 34: Monosynaptic Reflex Arc number of synapses. 3. At a synapse, the axonal tip or end foot releases a chemical called a neurotransmitter. The XI. SIX TENETS OF neurotransmitter crosses the synaptic cleft to THE NEURON DOCTRINE attach to receptor sites on the postsynaptic - Asserts that the neuron is the anatomic, functional, membrane that alters its electrical polarization. directional, genetic, pathologic, and regenerative A. Excitatory neurotransmitter depolarizes unit of the nervous system. the postsynaptic cell thereby promoting 1. Anatomic unit - Cell membrane separates it from impulse generation other neurons. B. Inhibitory transmitters hyperpolarize the 2. Functional unit - Smallest unit capable of postsynaptic cell, thereby opposing receiving, generating, and transmitting nerve impulse generation impulses. TABLE 4. NEUROTRANSMITTERS 3. Directional unit - Conducts impulses in only one Excitatory Glutamate, Acetylcholine direction, from dendrite to axonal tip. neurotransmitters (Ach), Histamine, Dopamine, 4. Genetic unit - Each neuroblast develops by mitosis Norepinephrine (NE), also from a primitive precursor that differs in structure, known as noradrenaline (NAd), biochemistry, and connections. Epinephrine (Epi) also known 5. Each neuron is a pathologic unit - Any part of a as adrenaline (Ad) neuron separated from its perikaryon dies, although Inhibitory Gamma-aminobutyric acid the rest of the cell may live. If the perikaryon itself neurotransmitters (GABA), Serotonine (5-HT), dies, all branches die. But, if only the axon dies it Dopamine (DA) still has the capacity to communicate with other Neuromodulators Dopamine, Serotonin, neurons. Acetylcholine, Histamine, Because of genetic differences in biochemistry Norepinephrine (NE) and structure, the various species of neurons differ in susceptibility to disease. Neurohormones Releasing hormones from Almost endless number of genetic or toxic (from hypothalamus, Oxytocin (Oxt), diseases that cause systematic degeneration hypothalamus) Vasopressin, also known as of specific types of neuron. antidiuretic hormone (ADH) 6. Each neuron is a regenerative unit - Most mature 4. The simplest communication loop, called a reflex neurons cannot multiply, but some can regenerate arc, involves an efferent neuron that responds to a axons (so long as the body/ perikaryon is alive) stimulus, one central synapse with an interneuron After transection of an axon, the distal part, that fires an efferent neuron that then activates an severed from its perikaryon dies, a process effector. called Wallerian degeneration. The surviving neuronal perikaryon of a peripheral axon may regenerate its severed axon. Peripheral nerve fibers therefore can effectively and functionally regenerate, but effective axonal regeneration of severed major tracts of CNS axons (e.g., spinal cord) in humans apparently does not occur. WALLERIAN DEGENERATION Figure 33: The Functions of a Reflex Arc Summary: The sensation of the sensory part will travel the peripheral nerve then to the spinal nerve then to the sensory ganglion in the dorsal root ganglion and that will send the information toward the dorsal horn of the spinal and it will not ascend but rather there will be an interneuron (association neuron) that will relay to the motor neuron to affect an action Figure 35: Wallerian Degeneration Batch 2028 1F 10 NEUROSCIENCE LC 1 An active process of anterograde generation of the distal end of an axon that is a result of a nerve lesion. It occurs between 7 to 21 days after the lesion occurs. After the 21st day, acute nerve degeneration will show on the electromyograph. XII. TYPES OF NEURONS 1. Functional classification sensory motor interneurons (association neurons) 2. Anatomical classification (by the number of processes) multipolar unipolar/pseudounipolar bipolar 3. Size/length classification of axon golgi type I golgi type II XIII. FUNCTIONAL CLASSIFICATION OF NEURONS 1. Sensory (Afferent) Related to the transfer of sensory information Figure 36: Somatomotor Integration (i.e., pain, touch, temperature, pressure, vibration) o Sensory (postcentral gyrus) cortex o Neurons of dorsal root ganglia Receive sensory input Conduct impulses towards the CNS 2. Motor (Efferent) Related to innervation of muscle, glands Activation of these neurons leads to some motor event (i.e.) contraction of a muscle) o Motor (precentral gyrus) cortex Conducts impulses from the CNS to the muscles and glands 3. Interneurons (Association Neurons) Figure 37: Interneurons: role in spinal reflex arc (e.g., patellar Neither motor or sensory reflex) Connects sensory and motor cortex Neurons responsible for the various spinal reflexes, connecting the neurons in the XIV. NEUROANATOMY BASICS posterior horns (sensory) to the anterior horn (motor) of the spinal cord Interconnections Establish neuronal circuit between sensory and motor neurons Table 5. Comparison of structural classes of neurons Batch 2028 1F 11 NEUROSCIENCE LC 1 Fig 39. Multipolar neuron (pyramidal cells of Bitz) > its axons bundle together to form the corticospinal tract (aka pyramidal tract) Table 6. Comparison of structural classes of neurons Table 7. Comparison of structural classes of neurons A. ANATOMICAL CLASSIFICATION OF NEURONS 1. MULTIPOLAR NEURONS Interneurons (integrates motor and sensory impulses in the CNS) Motor neurons (efferent pathway from CNS to Fig 41. Multipolar neurons (interneurons) acts as a conduit effector muscles or glands) between 2 neurons Efferent neurons 2. BIPOLAR NEURONS Sensory neurons Afferent pathway from visual/olfactory inputs towards the CNS Found in retina and olfactory epithelium Afferent neurons 3. UNIPOLAR/PSEUDOUNIPOLAR Primary/first order sensory neurons Sensory ganglia: dorsal root ganglia of cranial nerve ganglias No dendrites Afferent neurons Fig 40. Bipolar neuron Fig 38. Multipolar neurons (pyramidal cells of Betz) Fig 42. Bipolar neuron Batch 2028 1F 12 NEUROSCIENCE LC 1 Fig 45. Motor Pathway 2. GOLGI TYPE II Neurons is characterized by having short, or sometimes no, axons Fig 43. Pseudounipolar neurons Have smaller cell bodies than do type I neurons Do not send processes from one part of the nervous system to another. Rather, their processes are usually confined to a single nucleus or layer. Golgi type II neurons are involved in local interactions between nerve cells and are often called association neurons Dendrites and axons of type II neurons are often both pre and postsynaptic. That is, the dendrites are not exclusively receiving input, nor are the axons exclusively providing output, as in Golgi Type I neurons. Rather, both dendrites and axons can both receive and make synaptic contacts Cerebral and cerebellar cortex mostly functions as association neurons. Act locally E.g., within the confines of the cortex only and do not go beyond-ex. Stellate cells Interneurons/association neurons o Send information between sensory Fig 44. Pseudounipolar neuron neurons and motor neurons. Most interneurons are located in the central B. SIZE/LENGTH CLASSIFICATION OF AXON nervous system 1. GOLGI TYPE I Anaxonic neurons cannot communicate with other Long axon neurons that carry information neurons confined within the cerebral cortex. from one part of the brain to another (that is, from one nucleus or nuclear layer to another), or from the brain or spinal cord to effector organs such as muscles. They are larger than Golgi type II neurons Primary, secondary, and tertiary dendrites can be distinguished and the finest dendrites often have small lollipop-shaped on them. On the spines can be found much of the synaptic input to many Golgi Type I cells Each neuron has just one axon, which, as it comes off the cell body, is thinner than the primary dendrites. It remains roughly the same size throughout its length, and usually Fig 46 branches, if at all, only a few times near its termination XV. PROJECTION FIBERS Virtually all of the input to Golgi Type I neurons Consists of afferent and efferent of the cerebral is onto the dendrites or cell bodies and output cortex occurs at the axon terminals (one way Deeper to the cortex, these fibers are arranged information transfer). Dendrites transmit radially as the corona radiata information to the cell body, and axons transmit Then the fibers converge downward, form messages away from the cell body internal capsule, between thalamus and basal Pyramidal cells of the cerebral hemisphere ganglia Purkinje cells of the cerebellar cortex Continue in the crus cerebri of the midbrain, Motor cells of the spinal cord basilar part of pons, and pyramid of medulla Carry information from one part of the CNS to oblongata another Batch 2028 1F 13 NEUROSCIENCE LC 1 Fig 47: Projection Fibers Association fibers provide communication in a single hemisphere of the brain, either the right or the left side. Commissural fibers on the other hand provide connection between the left and the right side of the brain, as in the corpus callosum for example. BASIS FOR CLASSIFICATION EXAMPLE FUNCTIONAL IMPLICATION 1. Axonal projection Projection neuron or Principal Goes to a distant brain area. neuron or Golgi Type I cell (cortical Affects different brain areas motor neuron) Stays in a local brain area Intrinsic neuron or interneuron or Affects only nearby neurons Golgi type II cell (cortical inhibitory neurons 2. Dendritic pattern Pyramid-shaped spread of Pyramidal cell (hippocampal Large area for receiving synaptic input; dendrites pyramidal neuron) determines the pattern of incoming axons that can interact with the cell (ie. Pyramid-shaped) Radial-shaped spread of Stellate cell (cortical stellate cell) Large are for receiving synaptic input; dendrites determine the pattern of incoming axons that can interact with the cell (ie. Star-shaped) 3. Number of processes One process exits the cell Unipolar neuron (dorsal root Small area for receiving synaptic input: body ganglion cell) highly specialized function. Two processes exit the cell Bipolar neuron (retinal bipolar cell) Small area for receiving synaptic input: body highly specialized function Many processes exit the cell Multipolar neuron (spinal motor Large area for receiving synaptic input: body neuron) determines the pattern of incoming axons that can interact with the cell Reference: Viado, A. (August 6, 2024). Introduction to Neuroscience. Batch 2028 1F 14